DCDC switching converters are being designed to supply more and more complicated and highly integrated System-on-Chip (SoC) designs with fast transient load response being crucial for efficient operation and optimum performance. Various techniques have been proposed to improve the transient response by enhancing the speed of the controller response. Fast non-linear (hysteretic) control provides a close to ideal response, but typically remains limited due to the inductor slew rate. Multiphase topologies may be used to increase the slew rate of the (effective) inductor, at the expense of using a plurality of relatively bulky inductors that increase both volume and cost.
Another way to improve transient load response (at a given inductor current ripple) is using increased switching frequencies. Acceptable converter efficiency at increased switching frequencies requires the use of switches with improved figure-of-merit (FOM, which depends on the capacitance Cg and the resistance Rsp of the switch) as switching loss scales with frequency, therefore resulting in switches with reduced voltage rating. The reduced voltage rating of the switches typically makes the direct supply from a Lithium-Ion (Lilon) battery pack or a standard 5V or 12V power supply bus impossible. Therefore, a cascaded conversion via voltage pre-regulation towards an intermediate bus level (e.g. 1.8V) is typically used. Such double conversion of power typically reduces the overall conversion efficiency and increases solution cost.
Increased inductor current slew rate (via a reduced effective inductance) can also be achieved by multi-level conversion. The inductor current ripple during steady state operation is reduced by distributing the input-to-output voltage drop across the serial connection of e.g. a capacitor and an inductor, thereby enabling reduced inductance in comparison to a pure inductor based buck converter. During transient load conditions, the full input-to-output voltage drop may be applied to the reduced inductance, thereby resulting in an increased inductor current slew rate.
Hybrid schemes that comprise the parallel operation of a DCDC converter and a linear regulator can further improve the transient load response by injecting additional charging current. However, depending on the input-to-output voltage conversion ratio, the linear regulator may be exposed to a relatively large delta voltage especially when providing relatively high current. This can trigger relatively large voltage-times-current products, which in case of frequent load changes may impact the converter reliability, overall conversion efficiency and application heat-up.